Adhesives have been used for a variety of marking, holding, protecting, sealing and masking purposes. Adhesive tapes generally comprise a backing, or substrate, and an adhesive. One type of adhesive which is particularly preferred for many applications is represented by pressure sensitive adhesives.
Pressure-sensitive tapes are virtually ubiquitous in the home and workplace. In its simplest configuration, a pressure-sensitive tape comprises an adhesive and a backing, and the overall construction is tacky at the use temperature and adheres to a variety of substrates using only moderate pressure to form the bond. In this fashion, pressure-sensitive tapes constitute a complete, self-contained bonding system.
Pressure sensitive adhesives (PSAs) are well known to one of ordinary skill in the art, and according to the Pressure-Sensitive Tape Council, PSAs are known to possess properties including the following: (1) aggressive and permanent tack, (2) adherence with no more than finger pressure, (3) sufficient ability to hold onto an adherend, and (4) sufficient cohesive strength. Materials that have been found to function well as PSAs include polymers designed and formulated to exhibit the requisite viscoelastic properties resulting in a desired balance of tack, peel adhesion, and shear holding power. PSAs are characterized by being normally tacky at room temperature (e.g., 20° C.). PSAs do not embrace compositions merely because they are sticky or adhere to a surface.
These requirements are assessed generally by means of tests which are designed to individually measure tack, adhesion (peel strength), and cohesion (shear holding power), as noted in A.V. Pocius in Adhesion and Adhesives Technology: An Introduction, 2nd Ed., Hanser Gardner Publication, Cincinnati, Ohio, 2002. These measurements taken together constitute the balance of properties often used to characterize a PSA.
With broadened use of pressure-sensitive tapes over the years, performance requirements have become more demanding. Shear holding capability, for example, which originally was intended for applications supporting modest loads at room temperature, has now increased substantially for many applications in terms of operating temperature and load. When used as attachment devices for a variety of assembly and manufacturing applications, such as interior or exterior automotive mounting of panels and molding, or in the construction industry, pressure sensitive adhesives are additionally required to provide good adhesion performance to rough or irregular surfaces. In addition, many applications require pressure sensitive adhesives to support a load at elevated temperatures, typically in the range of from 70° C. to 90° C., for which high cohesive strengths are required. So-called high performance pressure-sensitive tapes are those capable of supporting loads at elevated temperatures for 10,000 minutes. Increased shear holding capability has generally been accomplished by crosslinking the PSA, although considerable care must be exercised so that high levels of tack and adhesion are retained in order to retain the aforementioned balance of properties.
In addition to increasing performance requirements with regard to pressure sensitive adhesives, volatile organic compounds (VOC) reduction regulations are becoming increasingly important in particular for various kind of interior applications (occupational hygiene and occupational safety) such as e.g. in the construction market or in the automotive or electronics industries. Known acrylate-based pressure sensitive adhesives typically contain notable amounts of low molecular weight organic residuals, such as un-reacted monomers arising from their polymerization process, polymerization initiator residuals, contaminations from raw materials or degradation products formed during the manufacturing process. These low molecular weight residuals qualifying as VOC may diffuse out of the adhesive tape and can be potentially harmful. Known acrylate-based pressure sensitive adhesives also generally suffer from lack of cohesive strength and excessive tendency to flow, when not crosslinked. This aspect may render the application and processability of uncrosslinked acrylate-based pressure sensitive adhesives particularly problematic, especially when made by a hotmelt process.
The reduction of organic solvent usage in the manufacturing process of pressure sensitive adhesives has quickly emerged as one straightforward means to reduce the overall VOC levels. The use of specific scavengers for organic contaminants, as described in WO 01/44400 (Yang), is another alternative way to achieve reduced VOC levels. However, the solutions for reducing overall VOC levels known from the prior art are often associated with increased manufacturing complexity and production costs.
It is therefore a recognized challenge in the adhesive tapes industry to combine good adhesion, good cohesion properties and low VOC levels. In order to optimize the adhesion of a PSA to a particular substrate, in particular an irregular substrate, an excellent surface wetting is necessary.
Partial solutions have been described in the art, whereby a non- or very low crosslinked adhesive is applied to a surface and then post-cured, so that, after an adequate surface wetting, the cohesive strength can be built up. In that context, the so-called “semi-structural tapes” described e.g. in U.S. Pat. No. 5,721,289 (Karim et al.) have been used. These systems are based on post-curable epoxy functionalities and specifically require using a superacid which is activated by UV irradiation as triggering energy. However, these known systems show undesired moisture sensitivity as the superacid needed for UV-induced cationic curing or crosslinking of the epoxy functionalities decomposes to hydronium ion, rendering ineffective the ring-opening polymerization of epoxies. Other known post-curable systems are based on the so-called “DICY-chemistry” described e.g. in EP-A1-0798354, wherein an epoxy-amine curing reaction is triggered with heat. However, these systems require a continuous heating step so as to maintain the curing reaction until the curing or crosslinking step has been completed. Patent applications US 2001/0076493-A1 (Kavanagh et al.) and US 2011/0178248-A1 (Kavanagh et al.) disclose pre-adhesive compositions comprising an aziridine crosslinking agent.
The commonly known curing or crosslinking systems do not often provide industrially viable solutions for the production of pressure sensitive adhesives having acceptable characteristics. Without contesting the technical advantages associated with the curing or crosslinking systems known in the art for producing pressure sensitive adhesives, there is still a need for a pressure sensitive adhesive provided with an excellent and versatile balance of adhesive and cohesive properties, in particular on uneven or irregular substrates, whilst ensuring reduced overall VOC levels.
Other advantages of the pressure sensitive adhesives and methods of the invention will be apparent from the following description.